Abstract

IGNACIO, Baliani; OTAMENDI, Juan E; TIBALDI, Alina M  and  CRISTOFOLINI, Eber A. Geology and petrology of mafic-ultramafic body from Las Juntas, Valle Fértil, San Juan. Rev. Asoc. Geol. Argent. [online]. 2012, vol.69, n.1. ISSN 0004-4822.

An elongate body of mafic and ultramafic plutonic rock crops out in central Sierra de Valle Fértil. The igneous body was tectonically emplaced and its internal magmatic fabrics strike roughly north-south and dip to the east. Rocks in the body are different from their hosting plutonic rocks in showing a layered igneous structure associated with pristine cumulate textural features at all observational scales. The layered body is composed of gabbronorite cumulates, ultramafic cumulates, and massive plagioclaserich cumulates. The cumulate gabbroic section largely consists of olivine gabbronorites interalyered with amphibole pyroxene gabbronorites and amphibole-bearing plagioclaserich gabbros (anorthosites). The cumulate ultramafic banks are largely composed of peridotites and dunites that contain varying proportions of pyroxenes, chromerich spinels, amphibole and small proportion of plagioclase. The cogenetic relation between mafic and ultramafic rocks is inferred from the Mg-Fe silicates compositions among all these rocks. The Fe-Mg exchange coefficient olivine-liquid and clinopyroxene-liquid reflect that this magma had Mg/Mg + Fe+² ratio of around 0.6; so that the magma had Mg/Mg + Fe+² ratio lower than that of a peridotiticmantle-derived primary magma. By implication, the main conclusion of this study is that primitive primary magmas originated at the lithospheric mantle - lower crust boundary zone ascended to feed chambers emplaced at upper levels within the lower crust (20 - 25 Km). This petrologic process is typical and characteristic of subduction-related magmatism, which in this case was related to the Famatinian arc.

Keywords : Cumulate rocks; Coronas; Igneous crystallization; Famatinian arc; Sierras Pampeanas.

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